Most power system monitoring, protection, and control functions may be performed efficiently and accurately if power system measurements at multiple locations are synchronized. However, it may be generally difficult to accurately synchronize clocks separated by large distances. Conventional techniques to synchronize data or clocks may have different delays in different directions between a pair of locations which may lead to an error in data or clock synchronization.
An existing data or clock synchronization technique for current differential protection may use echo or ping-pong, which assumes that the sending and receiving delays are same. However, the reliability of this technique may depend on the symmetry of communication links. For example, the delays of sending and receiving communication links may differ due to change in the communication routing. In other conventional technique, global positioning system (GPS) is used for data or clock synchronization. However, the signal of GPS may not be always reliable enough to meet the requirement of the current differential protection system.
In addition to being important for multi-terminal power transmission, clock synchronization is important in many other applications such as power relays, determinations of sequences of events, economic power dispatch, and the like. Facilitating communications between various terminals at different locations is one solution; however, the main challenge in facilitating communications may be caused by a clock rollover. Generally the clocks utilized may be within a limited range to save communication bandwidth. The limited range may result in a clock rollover which may cause multi-terminal clocks to converge to a stable but non-synchronized condition.
In some solutions, three terminals may be connected in a ring topology or mesh topology such that protection may continue even if communications failed (for example, due to a fault in the communication link) between one pair of terminals. In one such solution, the synchronization may be achieved by averaging the computed time shifts at each terminal. However, for the ring/mesh topology, multi-terminals may present challenges for clock synchronization since every terminal must have timing information from both its neighbors. Therefore, if a communications link fails between a pair of terminals, the timing information may have to be rerouted through other terminal in the ring/mesh topology. However, rerouting time message may result in delay in transmitting and receiving messages, and may require additional message forwarding and associated complexity of coding.
Alternatively, in another synchronization solution for the ring/mesh topology, each terminal may synchronize to only one of its neighbors. Although this solution may be implemented for synchronizing three terminals; however, it may not be implemented in case of four or more terminals due to the formation of “synchronization islands.” For example, in case terminals “A,” “B,” “C” and “D” are arranged in a ring communications topology, terminals A and B may synchronize to each other, and terminals C and D may synchronize to each other. However, in this example, the A-B synchronized pair may not synchronize with the C-D synchronized pair, thus resulting in “synchronization islands.”